Method and system for crosstalk cancellation

ABSTRACT

Signals propagating in one communication channel can generate crosstalk interference in another communication channel. A crosstalk cancellation device can process the signals causing the crosstalk interference and generate a crosstalk cancellation signal that can compensate for the crosstalk when applied to the channel receiving crosstalk interference. The crosstalk cancellation device can include a model of the crosstalk effect that generates a signal emulating the actual crosstalk both in form an in timing. The crosstalk cancellation device can include a controller that monitors crosstalk-compensated communication signals and adjusts the model to enhance crosstalk cancellation performance. The crosstalk cancellation device can have a mode of self configuration or calibration in which defined test signals can be transmitted on the crosstalk-generating channel and the crosstalk-receiving channel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 60/494,072, entitled “Method for CrosstalkCancellation in High-Speed Communication Systems,” and filed Aug. 7,2003. The contents of U.S. Provisional Patent Application Ser. No.60/494,072 are hereby incorporated by reference.

This application is related to U.S. Nonprovisional Patent ApplicationSer. No. 10/108,598, entitled “Method and System for Decoding MultilevelSignals,” filed on Mar. 28, 2002, and U.S. Nonprovisional PatentApplication Ser. No. 10/620,477, entitled “Adaptive Noise Filtering andEqualization for Optimal High Speed Multilevel Signal Decoding,” filedon Jul. 15, 2003. The contents of U.S. patent application Ser. No.10/108,598 and U.S. patent application Ser. No. 10/620,477 are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of communications, and morespecifically to improving the signal fidelity in a communication systemby compensating for crosstalk interference that occurs between two ormore communication channels that is prevalent at high data communicationrates.

BACKGROUND

Increased consumption of communication services fuels a need forincreased data carrying capacity or bandwidth in communication systems.A phenomenon known as crosstalk often occurs in these communicationsystems and can impair high-speed signal transmission and thus limitcommunication bandwidth to an undesirably low level.

Crosstalk is a condition that arises in communications systems wherein asignal in one communication channel is corrupted by interference (orbleed-over) from a different signal being communicated over anotherchannel. The interference may arise due to a variety of effects. Forexample, in electrical systems such as circuit boards, electricalconnectors, and twisted pair cable bundles, each electrical path servesas a channel. At high communication speeds, these conductive pathsbehave like antennae, both radiating and receiving electromagneticenergy. The radiated energy from one channel, referred to herein as the“aggressing channel,” is undesirably coupled into or received by anotherchannel, referred to herein as the “victim channel.” This undesirabletransfer of signal energy, known as “crosstalk,” can compromise dataintegrity on the receiving channel. Crosstalk is typically bidirectionalin that a single channel can both radiate energy to one or more otherchannels and receive energy from one or more other channels.

Crosstalk is emerging as a significant barrier to increasing throughputrates of communications systems. When not specifically addressed,crosstalk often manifests itself as noise. In particular, crosstalkdegrades signal quality by increasing uncertainty in the received signalvalue thereby making reliable communications more difficult, i.e. dataerrors occur with increased probability. In other words, crosstalktypically becomes more problematic at increased data rates. Not onlydoes crosstalk reduce signal integrity, but additionally, the amount ofcrosstalk often increases with the bandwidth of the aggressing signal,thereby making higher data rate communications more difficult. This isparticularly the case in electrical systems employing binary ormulti-level signaling, since the conductive paths over which suchsignals flow radiate and receive energy more efficiently at the highfrequencies associated with the level transitions in these signals. Inother words, each signal in a binary or multi-level communication signalis composed of high-frequency signal components that are moresusceptible to crosstalk degradation compared to lower frequencycomponents.

The crosstalk impediment to increasing data throughput rates is furthercompounded by the tendency of the high-frequency content of the victimsignal to attenuate heavily over long signal transmission path lengths(e.g. circuit traces that are several inches in length for multi-gigabitper second data rates). That is, high-frequency components of acommunication signal not only receive a relatively high level ofcrosstalk interference, but also are susceptible to interference becausethey are often weak due to transmission losses.

While these attenuated high-frequency components can be amplified via atechnique known as channel equalization, such channel equalizationfrequently increases noise and crosstalk as a byproduct of amplifyingthe high-frequency signals that carry data. The amount of crosstalkpresent in a communication link often limits the level of equalizationthat can be utilized to restore signal integrity. For example, at themulti-gigabit per second data rates desired for next-generationbackplane systems, the level of crosstalk energy on a communicationchannel can exceed the level of victim signal energy at the highfrequencies that underlie such high-speed communication. In thiscondition, extraneous or stray signal energy can dominate the energy ofthe desirable data-carrying signals, thus rendering communicating atthese data rates impractical with most conventional systemarchitectures.

The term “noise,” as used herein, is distinct from crosstalk and refersto a completely random phenomenon. Crosstalk, in contrast, is adeterministic, but often unknown, parameter. The conventional artincludes knowledge that it is theoretically possible to modify a systemin order to mitigate crosstalk. In particular, with definitions of: (i)the data communicated over an interfering or aggressing channel; and(ii) the signal transformation that occurs in coupling from theaggressing channel to the victim channel, the crosstalk can betheoretically determined and cancelled. That is, those skilled in theart understand that crosstalk signal degradation can be cancelled if thedata carried by a communication signal that is input into acommunication channel is known and the signal transformation imposed onthe communication signal by crosstalk is also known. However, achievinga level of definition of this signal transformation having sufficientprecision and accuracy to support a practical implementation of a systemthat adequately cancels crosstalk is difficult with conventionaltechnology. Consequently, conventional technology that addressescrosstalk is generally insufficient for high-speed (e.g. multi-gigabitper second) communications systems. Thus, there is a need in the art tocancel crosstalk so as to improve victim signal fidelity and remove thebarrier that crosstalk often poses to increasing data throughput rates.

While the physics giving rise to crosstalk (e.g. electromagneticcoupling in electrical systems or four-wave-mixing in optical systems)is well understood, this understanding alone does not provide direct andsimple models for the crosstalk transfer function. One common reason forconventional modeling difficulties is that the relative geometries ofthe victim and aggressor signal paths heavily influence the transferfunction of the crosstalk effect, and these paths can be quiteconvoluted. In other words, signal path complexity typically checksefforts to model crosstalk using conventional modeling methods based onanalyzing signal conduits. Furthermore, it is generally undesirable todesign a crosstalk canceller for a predetermined specific crosstalkresponse since: (i) a system may have many different responses fordifferent victim-aggressor pairs (each requiring a specific design); and(ii) different systems may need different sets of designs. Thus, thereis a need in the art for a crosstalk cancellation system and method withsufficient flexibility to: (i) accommodate the variety of crosstalktransfer functions that can stem from ordinary operations of a givensystem; and (ii) self-calibrate in order to avoid a complex manual taskof characterizing and adjusting for each victim-aggressor pair.

While the general concept of crosstalk cancellation is known in theconventional art, conventional crosstalk cancellation is typically notapplicable to high-speed environments, such as channels supportingmulti-gigabaud rates. Conventional crosstalk cancellation is typicallyimplemented in an entirely digital environment, wherein the accessibleaggressing data signals and received victim signals are digitized, and amicroprocessor implements the cancellation processes. Theanalog-to-digital converters and microprocessors needed to implementthis digital crosstalk cancellation in a high-speed environment areusually excessively complex, resulting in unacceptable power consumptionand product cost.

To address these representative deficiencies in the art, what is neededis a capability for crosstalk cancellation compatible with high-speedenvironments but with low power consumption and reasonable productioncost. A capability is further needed for automatically calibrating orconfiguring crosstalk cancellation devices. Such capabilities wouldfacilitate higher data rates and improve bandwidth in diversecommunication applications.

SUMMARY OF THE INVENTION

The present invention supports compensating for signal interference,such as crosstalk, occurring between two or more communication channels.Compensating for crosstalk can improve signal quality and enhancecommunication bandwidth or information carrying capability.

A communication signal transmitted on one communication channel cancouple an unwanted signal, such as crosstalk, into another communicationchannel and interfere with communication signals transmitting on thatchannel. In addition to occurring between two channels, this crosstalkeffect can couple between and among multiple communication channels witheach channel imposing crosstalk on two or more channels and receivingcrosstalk from two or more channels. A channel can be a medium, such asan electrical conductor or an optical fiber that provides a signal path.A single optical fiber or wire can provide a transmission medium for twoor more channels, each communicating digital or analog information.Alternatively, each channel can have a dedicated transmission medium.For example, a circuit board can have multiple conductors in the form ofcircuit traces in which each trace provides a dedicated communicationchannel.

In another aspect of the present invention, a crosstalk cancellationdevice can input a crosstalk cancellation signal into a channelreceiving crosstalk interference to cancel or otherwise compensate forthe received crosstalk. The crosstalk cancellation signal can be derivedor produced from a signal that is propagating on another channel,generating the crosstalk. The crosstalk cancellation device can becoupled between the channel that generates the crosstalk and the channelthat receives the crosstalk. In this configuration, the crosstalkcancellation device can sample or receive a portion of the signal thatis causing the crosstalk and can compose the crosstalk cancellationsignal for application to the channel that is receiving the unwantedcrosstalk. In other words, the crosstalk cancellation device can tapinto the channel that is causing the crosstalk, generate a crosstalkcancellation signal, and apply the crosstalk cancellation signal to thechannel receiving crosstalk interference to provide crosstalkcancellation or correction.

In another aspect of the present invention, the crosstalk cancellationdevice can generate the crosstalk cancellation signal via a model of thecrosstalk effect. The model can generate the crosstalk cancellationsignal in the form of a signal that estimates, approximates, emulates,or resembles the crosstalk signal. The crosstalk cancellation signal canhave a waveform or shape that matches the actual crosstalk signal. Asetting or adjustment that adjusts the model, such as a set of modelingparameters, can define characteristics of this waveform.

The crosstalk cancellation signal can be synchronized with the actualcrosstalk signal. That is, the timing of the crosstalk cancellationsignal can be adjusted to match the timing of the actual crosstalksignal. A timing delay or other timing parameter can define the relativetiming or temporal correspondence between the crosstalk cancellationsignal and the actual crosstalk signal.

In another aspect of the present invention, the crosstalk cancellationdevice can implement modeling and timing adjustments so the crosstalkcancellation signal closely matches the actual crosstalk, therebyyielding effective crosstalk cancellation. A controller of the crosstalkcancellation device can monitor and analyze the output of the crosstalkcancellation device. That is, a controller can process thecrosstalk-cancelled signal, which is an improved communication signalthat results from applying the crosstalk cancellation signal to thechannel having crosstalk interference. The controller can vary themodeling parameters and the timing delay, individually or in unison, tominimize any residual crosstalk remaining after crosstalk cancellation.Adjusting the operations of the crosstalk cancellation device cancompensate for fluctuating conditions and variations in the crosstalkeffect.

In another aspect of the present invention, a crosstalk cancellationdevice can undergo a calibration or setup procedure that is initiatedinternally or externally. The crosstalk cancellation device, or anotherdevice executing the calibration procedure, can initiate thetransmission of a known or predetermined test signal on a communicationchannel. A test signal can be transmitted on the channel that causescrosstalk or the channel that receives crosstalk interference. Also, onetest signal can be transmitted on a channel generating crosstalk, whilea different test signal is transmitted on a channel that receives thegenerated crosstalk interference. For example, a randomizedcommunication signal can propagate on the crosstalk-generating channel,while the crosstalk-receiving channel can have a uniform voltage orcurrent signal that is representative of essentially no datatransmission. The crosstalk cancellation device can utilize these knownconditions to define the timing and shape of a crosstalk cancellationsignal that effectively compensates for crosstalk interference. In otherwords, the crosstalk cancellation device can define or refine its modelof the crosstalk effect based on operating the crosstalk cancellationdevice with test signals transmitting on the crosstalk-generating andthe crosstalk-receiving communication channels.

The discussion of correcting crosstalk presented in this summary is forillustrative purposes only. Various aspects of the present invention maybe more clearly understood and appreciated from a review of thefollowing detailed description of the disclosed embodiments and byreference to the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a functional block diagram of a communication systemhaving two linecards communicating over a backplane and incurringcrosstalk.

FIG. 2 illustrates a functional block diagram of a crosstalk model forthe system illustrated in FIG. 1.

FIG. 3 illustrates a plot of a crosstalk response for abackplane-to-linecard connector according to an exemplary embodiment ofthe present invention.

FIG. 4 illustrates a functional block diagram of a crosstalkcancellation system according to an exemplary embodiment of the presentinvention.

FIG. 5 illustrates a functional block diagram of a crosstalkcancellation system including functional blocks of a crosstalkcancellation device according to an exemplary embodiment of the presentinvention.

FIG. 6 is a functional block diagram of a tapped delay line filteraccording to an exemplary embodiment of the present invention.

FIG. 7 is a functional block diagram of a crosstalk modeling filter of acrosstalk cancellation device with an adjustable delay according to anexemplary embodiment of the present invention.

FIG. 8 is a functional block diagram of a crosstalk modeling filter of acrosstalk cancellation device with a high-pass filter according to anexemplary embodiment of the present invention.

FIG. 9 is a functional block diagram of a control module of a crosstalkcancellation device according to an exemplary embodiment of the presentinvention.

FIG. 10 is a flow chart illustrating a process for canceling crosstalkaccording to an exemplary embodiment of the present invention.

FIG. 11 is a flow chart illustrating a process for calibrating acrosstalk cancellation device according to an exemplary embodiment ofthe present invention.

FIGS. 12A and 12B respectively illustrate testing data of acommunication system before and after implementing crosstalkcancellation according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention supports canceling crosstalk on one or morecommunication paths in a communication system, such as a high-speeddigital data communication system. A flexible and adaptable model of thecrosstalk effect can output a cancellation signal that accuratelyrepresents crosstalk interference. Coupling this cancellation signalonto a signal path that has crosstalk can cancel such crosstalk andthereby negate the impairment that crosstalk can impose on bandwidth.

Turning now to discuss each of the drawings presented in FIGS. 1–12B, inwhich like numerals indicate like elements throughout the severalfigures, an exemplary embodiment of the present invention will bedescribed in detail.

Turning now to FIG. 1, this figure illustrates a functional blockdiagram of a communication system 100 having two linecards 101 a, 101 bcommunicating over backplane signal paths 120, 130 exhibiting crosstalk150, 151. More specifically, FIG. 1 illustrates an occurrence ofbackplane crosstalk 150, and connector crosstalk 151 in the exemplarycase of a backplane communications system 100.

A linecard 101 a, 101 b is a module, typically a circuit board thatslides in and out of a chassis slot and provides communicationcapabilities associated with a communication channel. A backplane 103 isa set of signal paths, such as circuit traces, at the rear of such achassis that transmit signals between each installed linecard 110 a, 101b and another communication device, such as another linecard 110 a, 101b or a data processing component in a rack-mounted digital communicationsystem.

Each linecard 110 a, 101 b in the system 100 illustrated in FIG. 1transmits and receives multiple channels of data, such as the twoillustrated channels 120, 130. An exemplary channel 130: (i) starts at atransmitter (Tx) 104 a on a linecard 101 a; (ii) transmits off thelinecard 110 a through a connector 102 a to the backplane 103; (iii)continues across the backplane 103 to another connector 102 b andlinecard 101 b; and (iv) is received by a receiver (Rx) 105 b. FIG. 1shows two such channels labeled the “victim” or “vict.” (from victimtransmitter 104 a to victim receiver 105 b) and the “aggressor” or“agg.” (from aggressor transmitter 104 b to aggressor receiver 105 a).

When the signal paths 120, 130 are in close proximity to one another,signal energy radiates from the aggressor channel 120 and isincorporated into the victim channel 130. That is, in areas of thebackplane 103 and connectors 102 a, 102 b in which a first signal pathis located close to a second signal path, a portion of the signal energypropagating in the first signal path can couple into the second signalpath and corrupt or impair signals propagating on this second signalpath. This crosstalk coupling 150 may occur on a linecard 110 a, 101 b,in a connector 102 a, 102 b, on a backplane 103, or any combinationthereof, for example.

While not illustrated in FIG. 1, crosstalk can also occur in the reversedirection. Specifically, the “victim” channel 130 often radiates energywhich corrupts the “aggressor” channel 120. That is, crosstalkfrequently occurs in a bidirectional manner, transferring not only froma first signal path to a second signal path but also from the secondsignal path to the first signal path. Furthermore, in systems havingthree or more signal paths coexisting in close proximity to one another(not illustrated), crosstalk can transfer among and between three ormore signal paths. That is, a single signal can not only imposecrosstalk on two or more other signals, but also receive crosstalkinterference from two or more other signals.

Similar to the multi-physical-path case illustrated in FIG. 1 anddescribed above, crosstalk can occur with the aggressor and victimchannels propagating on a single transmission medium (e.g. a singlecable or trace). In this scenario, each channel can correspond to aparticular signal band (e.g. a frequency band in frequency divisionmultiplexing system, a spectral band as in an optical wavelengthdivision multiplexing system, or a temporal window in time-divisionmultiplexing system). In other words, two communication channels, onegenerating crosstalk, one receiving crosstalk, can coexist in acommunication medium such as an optical waveguide or a wire, with eachcommunication channel supporting transmission of a dedicatedcommunication signal.

For clarity of explanation, an exemplary embodiment of the presentinvention based on crosstalk occurring between two channels, each on aseparate physical path, is illustrated in FIG. 1 and described in detailherein. In another exemplary embodiment of the present invention, amethod and system cancels crosstalk occurring between channelscoexisting on a single communication medium. One skilled in the artshould be able to make and use the present invention in an applicationhaving two or more channels exhibiting crosstalk on a singlecommunication medium following the detailed description, flow charts,plots, and functional block diagrams included herein.

Turning now to FIG. 2, this figure illustrates a functional blockdiagram 200 of a crosstalk model 210 for the system 100 illustrated inFIG. 1. More specifically, FIG. 2 illustrates a model 210 of thecrosstalk effect 151 in the connector 102 b based on a single, exemplarytransfer function 210.

The aggressor transmitter 104 b outputs an aggressor communicationsignal u(t) 215 on the aggressor channel 120. Energy from this aggressorcommunication signal u(t) 215 couples into the victim channel 130 viacrosstalk 151 in the connector 102 b. The aggressor communication signalu(t) 215 is composed of a spread of frequencies. Since crosstalk 151 isa frequency dependent phenomenon, the frequencies of the aggressorcommunication signal u(t) 215 couple into the victim channel at varyingefficiency. The frequency model H(f) 210 of the crosstalk effect 151expresses the extent to which each of these frequency components couplesinto the victim channel 130 in the form of a signal n(t) 230. Thiscrosstalk signal n(t) 230 combines with the unadulterated communicationsignal x(t) 214 propagating on the victim channel 130 from the victimtransmitter 104 a. The victim channel 130 transmits the resultingcombined signal y(t) 260 to the victim receiver 105 b.

The crosstalk transfer function 210 can be characterized by thefrequency response H(f) 210 (or its time-domain equivalent impulseresponse h(t)). As depicted in FIG. 2, the response H(f) 210 conveys thetransformation that the aggressor data signal u(t) 215 experiences inthe connector portion of its route from the aggressing transmitter 104 bto the victim receiver 105 b. The specifics of this response 210 usuallyvary among particular victim-aggressor channel pairs. Nevertheless, thegeneral nature of the response is based on geometric constraints andunderlying physics. For example, a backplane connector's crosstalkresponse 151 can depend upon physical system parameters. The backplanecrosstalk 150 can also be modeled with a transfer function, and thebackplane and connector crosstalks 150, 151 can even be captured with asingle (albeit different) transfer function.

An exemplary, non-limiting embodiment of the present invention, in whicha crosstalk cancellation device compensates for crosstalk occurring on alinecard-to-backplane connection will be described below with referenceto FIGS. 3–12B. The embodiments disclosed herein are provided so thatthis disclosure will be thorough and complete and will convey the scopeof the invention to those having ordinary skill in the art. Thoseskilled in the art will appreciate that the present invention can beapplied to address crosstalk occurring on a backplane or other locationsin a communication system and that the present invention can compensatefor various forms of crosstalk.

Turning now to FIG. 3, this figure illustrates a plot 300 of a crosstalkresponse 210 for a backplane-to-linecard connector 102 b according to anexemplary embodiment of the present invention. This plot 300 illustrateslaboratory measurements of the power in the crosstalk signal 151, morespecifically the power transferred from the aggressor channel 120 to thevictim channel 130 in the connector 102 b, as a function of frequency.The horizontal axis is frequency measured in units of gigahertz (GHz).The vertical axis describes signal power in decibels (“dB”), morespecifically ten times the base-ten logarithm of the crosstalk frequencyresponse 210 squared. Thus, this plot 300 illustrates the level ofcrosstalk power transferred from one channel 120 to another channel 130for each frequency component of the aggressor signal u(t) 215.

In connectors 102 a, 102 b, the dominant mechanism for crosstalk 151 istypically capacitive coupling between the connector's pins. Thismechanism is clearly evident in FIG. 3 as the general high-pass natureof the response of the plot 300. In other words, the plot 300 shows atrend of higher signal frequencies, above about 1 GHz, transferringenergy via crosstalk mechanisms 151 more readily than lower frequencies,below 1 GHz. The left side of the plot 300, less than approximately 1GHz, exhibits an attenuated crosstalk signal having a power less thanapproximately −25 dB. Thus, this plot 300 shows that the frequencycomponents less than approximately 1 GHz of a communication signal u(t)215 transfer a relatively small portion of their carried power to avictim channel 130 via connector crosstalk 151. The magnitude ofcrosstalk 151 increases between approximately 0.25 GHz and 1 GHz. Thus,based on this plot 300, the components of a victim communication signalx(t) 214 that have frequencies between approximately 1 GHz and 4.25 GHzare particularly prone to crosstalk effects 151 from an aggressorcommunication signal u(t) 215 with similar signal frequencies.

Furthermore, the fluctuations in the frequency response plot 300 atfrequencies above 2 GHz illustrate that the crosstalk effect 151 isheavily influenced by other effects than simple capacitive couplingbetween a pair of pins. In other words, above 2 GHz, the plot 300deviates from a classical capacitive coupling response, which typicallyasymptotically (and monotonically) increases with increased frequency.In contrast, the illustrated plot 300 exhibits a pattern of peaks andvalleys at higher frequencies, such as a local minimum at approximately4.6 GHz.

As described above, adequate crosstalk cancellation depends heavily onaccurately modeling a system's crosstalk response. Crosstalkcancellation performance is particularly dependent upon model accuracyfor frequencies in which the crosstalk effect is strong, namely forfrequencies above approximately 1 GHz.

The higher order effects of the aforementioned peaks and valleys in theplot 300 are highly dependent on specific relative geometric relationsbetween the victim signal path 130 and aggressor signal path 120 whichare not known a priori, in general. In other words, deriving an accurateand sufficient crosstalk model based on geometric or physical analysisof communication paths can be problematic without empirical data or testmeasurements regarding actual crosstalk impact on a signal.

Stated another way, the plot 300 of FIG. 3 illustrates that the higherfrequency components of a communication signal 214, 215 are particularlyprone to crosstalk 151 and that modeling the crosstalk response 210 forthese higher frequency components involves addressing the inherentlyerratic nature of this high frequency response. Since an accurate modelof a system's crosstalk response 210 can provide the basis for adequatecrosstalk cancellation, such a model needs to accurately represent thesehigher-order, erratic response characteristics. While passive circuitanalysis does not readily derive a model with requisite accuracy, actualsignal responses can serve as a basis for constructing an appropriatemodel.

In one exemplary embodiment of the present invention, a crosstalk modelin a crosstalk cancellation device can be defined based on crosstalkmeasurement data such as the measurement data presented in the plot 300illustrated in FIG. 3. As an alternative to acquiring such measurementdata in the laboratory, the data can be acquired during fieldoperations, for example by switching a crosstalk cancellation deviceinto a calibration mode as will be discussed below in reference to FIG.9 and FIG. 1.

Turning now to FIG. 4, this figure illustrates a functional blockdiagram of a crosstalk cancellation system 400 according to an exemplaryembodiment of the present invention. As described above, the presentinvention can provide crosstalk cancellation in high-speed digitalcommunication systems, such as the communication system 100 illustratedin FIGS. 1 and 2 and discussed above. More specifically, FIG. 4illustrates a crosstalk cancellation device or crosstalk canceller(“XTC”) 401 arranged to cancel crosstalk 151 occurring in abackplane-to-line card connector 101 b as discussed above with referenceto FIGS. 1, 2, 3, and 4.

Digital data x(t) 214 propagates in the victim channel 130 for receptionby the victim receiver 105 b. The victim channel 130 also carries theunwanted crosstalk signal n(t) 230 that is derived from digital datau(t) 215 output by the aggressor transmitter 104 b and that is notintended for reception at the victim receiver 105 b. The intended datastream signal x(t) 214 and the crosstalk signal n(t) 230 additively formthe composite signal y(t) 260. The crosstalk canceller 401 receives thecomposite signal y(t) 260, corrects the crosstalk interference n(t) 230from this signal 260 via cancellation and outputs a corrected signal,z(t) 420 for receipt by the victim receiver 105 b. That is, thecrosstalk canceller 401 applies an estimate of the actual crosstalk 230to the signals 260 propagating in the victim channel 130 to effectivelycancel the crosstalk signal elements 230 while leaving the desired datasignal 214 essentially intact.

The steps that the crosstalk canceller 401 performs include:

-   -   (i) accepting as separate inputs y(t) 260 (the victim signal        corrupted by crosstalk 151) and a representative portion of u(t)        215 (the aggressor signal propagating on the aggressor channel        120 giving rise to the crosstalk signal 230);    -   (ii) transforming the transmitted aggressor signal u(t) 215 into        crosstalk estimates that emulate the signal transformation 210        that actual occurs in the system 200 via the crosstalk effect        151;    -   (iii) subtracting the modeled crosstalk from the victim y(t) 260        to cancel its crosstalk signal n(t) 230 component; and    -   (iv) outputting the compensated signal z(t) 420 to the victim        receiver 105 b, which can be a conventional receiver without        specific technology for crosstalk compensation.

Turning now to FIG. 5, this figure illustrates a functional blockdiagram of a crosstalk cancellation system 500 according to an exemplaryembodiment of the present invention. More specifically, FIG. 5illustrates an architectural overview of an exemplary crosstalkcanceller 401 that has three functional elements 501, 502, 503, acrosstalk model 501, a summation node 502, and a 503 controller,electronic control “mechanism”, or control module. The model 501generates a crosstalk estimate signal w(t) 520, while the summation node502 applies this crosstalk estimate 520 to the victim channel 130. Thecontroller 503 adjusts parameters in the model 501 based on the outputz(t) 420 of the summation node 502.

The model 501 emulates the aggressor transfer function H(f) 210 in theform of an adjustable frequency response function G(f) 501. That is, themodel 501 generates an artificial crosstalk signal w(t) 520 that can bea model, simulation, estimate, or emulation of the actual, interferingcrosstalk signal n(t) 230 caused by electromagnetic coupling in theconnector 102 b between the aggressor channel 120 and the victim channel130. The model frequency response G(f) 501 effectively filters theaggressor data signal u(t) 215 in a manner that applies a frequencydependent response similar to the plot 300 illustrated in FIG. 3 anddiscussed above.

Because the same aggressor data stream u(t) 215 drives both the actualcrosstalk response H(f) 210 and the crosstalk canceller' model 501, theoutput w(t) 520 of the model 510 is, in the ideal case, equal to theaggressor signal component n(t) 230. That is, G(f) 501 equals H(f) 210in a theoretical or ideal case in which the environment is noise-freeand all system parameters are known and modeled perfectly. Furthermore,in this ideal scenario, the respective output signals n(t) 230 and w(t)520 of H(f) 210 and G(f) 501 would also be equal to one another. In areal-world situation having numerous unknown influences andindeterminate factors, G(f) 501 approximates H(f) 210 with sufficientprecision and accuracy to support essentially error-free communicationsof high-speed data rates.

The difference node 502 subtracts the emulated aggressor signal w(t)520, or emulation signal 520, from the composite signal y(t) 260, thusremoving or reducing crosstalk interference from the received victimsignal y(t) 260. In a physical implementation functioning in areal-world operating environment, the model G(f) 501 does not exactlymatch the true response H(f) 210. The controller 503 adjusts the model501 to minimize this error related to inaccuracies between the actualcrosstalk effect H(f) 210 and the emulated or modeled crosstalk effectG(f) 501.

Implementation of the summation node 502 is usually straightforward tothose skilled in the art. However, special attention should be paid tomaintain high sensitivity to the two inputs. It is not uncommon for theincurred, and thus the modeled, crosstalk signals 230, 520 to be smallin amplitude, especially at high frequencies. While seemingly negligibleat first glance, these high frequencies are often amplified viaequalization devices (not illustrated). Thus, while the neglectedhigh-frequency crosstalk may be small before equalization, it can bevery significant after equalization. The summation node should beimplemented to accommodate such high-frequency response.

A portion of the compensated signal z(t) 420 (i.e. the output of thedifference node 502) is tapped off and fed to the controller 503,providing the controller 503 with essentially the same signal 420 thatthe victim receiver 105 b receives. The controller adjusts theparameters of the modeling filter 501, characterized by the responseG(f) 501, to maximize the goodness-of-fit to the actual response H(f)210. In particular, the controller 503 takes as input the crosstalkcompensated signal z(t) 420 and processes, monitors, or analyzes thatsignal 420 to determine signal fidelity. In other words, the controller503 evaluates the model's performance by analyzing the extent to whichthe model's output 520 has cancelled the crosstalk signal 230. Thecontroller 503 also adjusts the model 501 to enhance crosstalkcancellation and to provide dynamic response to changing conditions.

Because the output of the controller 503 includes parameters of themodeling filter 501, the controller can adjust the modeled response G(f)420. Consequently, the controller 503 can manipulate the modeling filter501 to maximize the fidelity of the compensated signal 420, i.e. thematch between G(f) 420 and H(f) 210, by minimizing crosstalk on z(t)420. Stated another way, the controller 503 monitors the corrected,crosstalk-cancelled signal z(t) 420 and dynamically adjusts thecrosstalk model G(f) 420 to improve the crosstalk cancellation andenhance signal quality. Thus, in one exemplary embodiment of the presentinvention, a crosstalk cancellation device 401 can include a feedbackloop that adapts, self-corrects, or self configures crosstalkcancellation to compensate for modeling errors, fluctuating dynamicconditions, and other effects.

The system illustrated in FIG. 5 can be implemented primarily usinganalog integrated circuitry to provide a relatively low degree ofcomplexity, power consumption, and cost. In one embodiment, the model501 and difference node 502 are entirely analog. In another embodiment,certain aspects of the model 501 are implemented digitally to exploitthe digital nature of the aggressor data source 104 b.

The controller 503 typically includes both analog and digital circuitry.Due to particular aspects of the analog pre-processing in the controller503, this digital circuitry can operate at a low speed relative to thecommunication data rate and thus can facilitate practicalimplementation. In particular, the digital circuitry can operate atspeeds that are orders of magnitude less than the channel baud rate. Inone exemplary embodiment of the present invention, a digital circuit inthe controller 503 operates at least one order of magnitude below thechannel baud rate. In one exemplary embodiment of the present invention,a digital circuit in the controller 503 operates at least two orders ofmagnitude below the channel baud rate. In one exemplary embodiment ofthe present invention, a digital circuit in the controller 503 operatesat least three orders of magnitude below the channel baud rate. Furtherdetails exemplary embodiments of the controller 503 and the model 501that together yield a low-power and low-cost crosstalk cancellationsolution are discussed in more detail below.

Turning now to FIG. 6, this figure is a functional block diagram of atapped delay line filter 600 according to an exemplary embodiment of thepresent invention. A tapped delay line filter 600 is a device thatgenerates an output signal 620 from an input signal 215 by delaying theinput signal 215 through a series of delay stages 601 a, 601 b, 601 c;scaling the output of each delay stage 601 a, 601 b, 601 c, typicallywith an amplifier 602 a, 602 b, 602 c, 602 d; and adding or otherwisecombining these scaled outputs. The tapped delay line filter 600 can bean analog component of the model 501 that generates a signal v(t) 620having a shape or waveform approximating that of the imposed crosstalksignal n(t) 230. That is, the tapped delay line filter 600 can be anexemplary waveform shaper that is implemented via analog components.

As described above, accurately modeling the actual crosstalk response210 facilitates adequate removal of crosstalk interference 230 viacrosstalk cancellation. If a crosstalk cancellation device (notillustrated) were based on an inaccurate crosstalk model (notillustrated) such a device might degrade, rather than improve, signalquality. For example, as a result of an erroneous model, a “correction”signal intended to cancel crosstalk might add interference to a receivedvictim signal while leaving the crosstalk signal that is targeted forcancellation essentially intact. Thus, a crosstalk model, for examplebased on a filtering mechanism, should have sufficient flexibility tosupport modeling a variety of crosstalk transfer functions that may beencountered in an application. That is, a flexible crosstalk model isdesirable over a rigid model that cannot readily adapt to variousapplications, operating conditions, and environments, for example.

In one exemplary embodiment of the present invention, as illustrated inFIG. 6, an analog tapped delay line filter 600 (also known astransversal filter) with electrically controllable gain coefficients 602a, 602 b, 602 c, 602 d; models the aggressor crosstalk transfer function210. This filter 600 can provide a desirable level of flexibility andadaptability that supports a wide range of operating conditions andsituations. More specifically, the tapped delay line filter 600 cangenerate a waveform approximating the waveform of the crosstalk signal230 imposed on the victim channel 130.

The illustrated filter 600 is an exemplary tapped delay line filter withN delay elements 601 a, 601 b, 601 c (each providing time delay δ(delta)) and corresponding variable coefficient amplifiers 602 a, 602 b,602 c, 602 d with coefficients α_(n) (alpha_(n)) for n=0, . . . ,N. Theoutput v(t) 620 of the tapped delay filter 600 can be written asv(t)=α₀ u(t)+α₁ u(t−δ)+ . . . +α_(N) u(t−Nδ).

Changing the values of the gain coefficients α₀, α₁, α₂ . . . α_(n)(alpha₀, alpha₁, alpha₂ . . . alpha_(n)) can cause a correspondingchange in the response of the filter 600. The tapped delay line filter600 can model the aggressor's impulse response for up to Nδ (N timesdelta), that is, up to the temporal span of the filter 600.Additionally, the frequency content of the aggressor response 210 (asillustrated in FIG. 3 and discussed above) can be modeled up to afrequency of f=1/(2δ) (frequency equals the reciprocal of two timesdelta). Thus, δ (delta) should be chosen such that the highest signalfrequency of interest in the victim signal x(t) 214 is less thanf=1/(2δ) (frequency equals the reciprocal of two times delta).Furthermore, N should be chosen so that the majority of the aggressorimpulse response is contained within a temporal span of Nδ (N timesdelta). Equivalently, the aggressor frequency response 210 should notexhibit large fluctuations below frequencies of f=1/(Nδ) (frequencyequals the reciprocal of N times delta). These conditions for selectingN and δ (delta) contrast with the aggressor signal's conditions. It isnot critical if aggressor noise remains above the specified frequency,because a well designed receiver can readily suppress these higherfrequencies without degrading victim signal quality.

While a tapped delay line filter 600 can emulate, estimate, or mimicpulse shaping caused by the aggressor response 210, this filter 600typically cannot adequately address highly variable temporal delaywithout an unwieldy number of taps or delay stages. Temporal delay isdirectly associated with the length of the signal path that spansbetween (i) the circuit tap that directs a portion of the aggressor datasignal u(t) 215 to the crosstalk canceller 401 and (ii) the summationnode 502 in the crosstalk canceller 401, as illustrated in FIG. 5 anddiscussed above. More specifically, the modeled temporal delay shouldclosely approximate the temporal delay of the actual crosstalk signaln(t) 230 so that the modeled and actual signals 230, 520 are properlysynchronized or timed with respect to one another for effective mutualcancellation. While the output 620 of the tapped delay line filter 600can be directly used as the output w(t) 520 of the model 501,synchronizing the tapped delay line filter's output 620 with thecrosstalk signal 230 on the victim channel 130 can enhance crosstalkcancellation, provide heightened signal fidelity to the victim receiver105 b, and improve overall modeling flexibility.

Because the locations of the coupling points of both the actualcrosstalk signal 230 and its modeled counterpart 520 can besignificantly variable among victim-aggressor pairs, their respectivedelays can be ill-defined or subject to uncertainty. Even in therelatively simple case of dominant coupling via the backplane-linecardconnector 102 b, the signal path length on the linecard 101 b is oftenvariable. Thus, the temporal delay can be difficult to predict withoutspecific knowledge and analysis of linecard layout. To address thisuncertainty in temporal delay, an adjustable delay 701 can beincorporated into the cross talk modeling filter 501 as illustrated inFIG. 7.

Turning now to FIG. 7, this figure is a functional block diagram of acrosstalk modeling filter (“XTMF”) 501 of a crosstalk cancellationdevice 401 with an adjustable delay 701 according to an exemplaryembodiment of the present invention. The adjustable delay 701 can eitherprecede or succeed (as shown in FIG. 7) the tapped delay line filter600. In one exemplary embodiment of the present invention, placing theadjustable delay 701 on the input side of the analog tapped delay linefilter 600, rather than the output side as illustrated, can simplify theimplementation. This simplification can result from the discrete natureof the digital signal u(t) 215 wherein signal linearity can be readilymaintained by quantizing or hard-limiting the output of the delay device701. Alternatively, if the adjustable delay 701 follows the tapped delayline filter 600, in accord with the illustrated configuration, thesignal v(t) 620 is analog at the input to the adjustable delay 701.Inputting an analog signal into the adjustable delay 701 can impose aneed for a linear response over a broad range of signal values andfrequencies, which can be difficult to achieve for large delay values.

While the tapped delay line filter 600 outputs a correction signal w(t)520 that approximates the crosstalk signal n(t) 230 undesirablypropagating on the victim channel 130 alongside the intended data signalx(t) 214, the adjustable delay 701 synchronizes the waveform of thecorrection signal 520 with the waveform of the undesirable crosstalksignal 230. That is, the adjustable delay 701 times or coordinates thecorrection signal 520 so it temporally matches and is synchronized withthe actual crosstalk interference 230.

Based on the functions of the tapped delay line filter 600 and theadjustable delay 701, the crosstalk modeling filter 501 outputs acancellation signal w(t) 520 having form and timing accurately matchingthe actual crosstalk signal n(t) 230. When inserted into or applied tothe victim channel 130 via the subtraction node 502, as illustrated inFIG. 5 and discussed above, the cancellation signal w(t) 520 negates theactual crosstalk signal 230 and thereby enhances the quality of thecommunication signal z(t) 420 delivered to the victim receiver 105 b.

As discussed in further detail above with reference to FIG. 5 and belowwith reference to FIG. 8, the controller 503 adjusts the tapped delayline filter 600 and the adjustable delay 701 to fine tune theirrespective performances and to enhance the fidelity of the correctedsignal 420 delivered to the victim receiver 105 b.

Turning now to FIG. 8, this figure is a functional block diagram of acrosstalk modeling filter 501′ of a crosstalk cancellation device 800with a high-pass filter 801 according to an exemplary embodiment of thepresent invention. The high-pass filter 801 is typically a fixed ornon-adjustable filter. In the configuration of the exemplary embodimentof the illustrated in FIG. 8, the adjustable delay 701 feeds the tappeddelay line filter 600, thus offering advantages, as discussed above inreference to FIG. 7, for certain applications.

Including the optional high-pass filter 801 in the exemplary crosstalkmodeling filter 501′, as illustrated in FIG. 7, can enhance performancein some applications or operating environments. A high-pass filter 801is a device that receives a signal having a range of frequencycomponents, attenuates frequency components below a frequency threshold,and transmits frequency components above the frequency threshold.

While tapped delay line filters 600 have a flexible modeling responseover the frequency range1/(Nδ)<f<1/(2δ),they are often less flexible at lower frequencies such as f<1/(Nδ)(frequencies less than the reciprocal of two times delta). Thus,accurately modeling low-frequencies characteristics of the crosstalkresponse 210 may require a large number N of filter taps that increasefilter complexity or may require a longer delay increment δ (delta) thatreduces high-frequency flexibility. In many applications, it ispreferable to avoid such trade-offs. As discussed above with referenceto FIG. 3, for electrical systems, low-frequency crosstalkcharacteristics are usually dominated by capacitive coupling effects andcan consequently be accurately modeled with a high-pass filter such as asimple first-order resistor-capacitor (“RC”) high-pass filter. That is,inserting the high-pass filter 801 into the crosstalk modeling filter801 can provide a high level of performance without requiring acumbersome or expensive number of tap filters in the tapped delay linefilter 600.

Similar to the exemplary embodiment of the crosstalk modeling filter 501depicted in FIG. 7, the ordering of the tapped delay line filter 600,the adjustable delay 701, and high-pass filter 801 can be permuted tosupport various arrangements. That is, the present invention supportsarranging physical components corresponding to each of the functionalblocks 701, 600, 801 illustrated in FIG. 8 in any parallel or seriesconfiguration that provides acceptable performance for an intendedapplication. Nevertheless, certain configurations or ordering mayprovide certain advantages or tradeoffs for select applicationsituations as compared to other configurations.

The exemplary inline configuration illustrated in FIG. 8 places theadjustable delay 701 on the input side of the tapped delay line filter600 and the high-pass filter 801 on the output side of the tapped delayline filter 600. With this ordering, the implementation of theadjustable delay 701 can be simplified by exploiting thediscrete-amplitude nature of both its input and output signal. Thetapped delay line filter 600 can also exploit, via digital delayelements, the discrete-amplitude input provided from the adjustabledelay 701. In its RC implementation, the high-pass filter 801 is ananalog device that does not receive benefit from providing it with adiscrete amplitude input. Thus, there is typically no drawback toplacing the high-pass filter 801 at the output side of the crosstalkmodeling filter 501′ or in another position.

As discussed above with reference to FIG. 5, the control module 503takes as input the crosstalk compensated signal z(t) 420 and outputscontrol signals 820, 830 to adjust the crosstalk response model 501. Thecontrol module's outputs 820, 830 to the crosstalk modeling filter 501comprises: (i) a “delay control” signal 830 to control the time delayimplemented by the adjustable delay component 701; and (ii) a set of“filter control” signals 820 to control the gains on the variablecoefficient amplifiers 602 a–d in the tapped delay line filter 600. Thatis, the controller 503 outputs modeling parameters to the tapped delayline filter 600 and timing parameters to the adjustable delay 701.

These output control values are determined based on observation,processing, and/or analysis of the compensated signal z(t) 420. U.S.Nonprovisional patent application Ser. No. 10/108,598, entitled “Methodand System for Decoding Multilevel Signals” and filed on Mar. 28, 2002,discloses a viable exemplary system and method for assessing signalfidelity. Commonly owned U.S. Nonprovisional patent application Ser. No.10/620,477, entitled “Adaptive Noise Filtering and Equalization forOptimal High Speed Multilevel Signal Decoding” and filed on Jul. 15,2003, discloses a viable exemplary system and method for controllingdevice parameters of the crosstalk modeling filter 501. The disclosuresof U.S. patent application Ser. No. 10/108,598 and U.S. patentapplication Ser. No. 10/620,477 are hereby fully incorporated byreference. One or more of the crosstalk model 501, the tapped delay linefilter 600, and the adjustable delay 701 can each be controlled and/oradjusted using a method and/or system disclosed in U.S. patentapplication Ser. No. 10/108,598 or U.S. patent application Ser. No.10/620,477. The temporal delay adjustment of the adjustable delay 701can be determined by treating the delay control as a variable that isswept through its entire range of potential values following thedisclosure of these patent applications, for example.

Turning now to FIG. 9, this figure illustrates an exemplary system 900for controlling a crosstalk model 501 such as the exemplary crosstalkmodeling filter 501 illustrated in FIG. 8 or the exemplary crosstalkmodeling filter 501 illustrated in FIG. 7 and their associatedadjustable delays 701. More specifically, FIG. 9 is a functional blockdiagram of a control module 900 of a crosstalk cancellation device 401according to an exemplary embodiment of the present invention. Theexemplary controller 900 illustrated in FIG. 9 facilitates relativelysimple theoretical analysis and implementation and, in that regard, canoffer benefit to certain application over the control methods andsystems disclosed in U.S. patent application Ser. No. 10/620,477 andU.S. patent application Ser. No. 10/108,598 that are discussed above.

The controller 900 of FIG. 9 includes a filter 901, having a frequencytransfer response P(f), that receives the crosstalk-cancelled signalz(t) 420 destined for reception by the victim receiver 105 b. Filter 901can be a spectral weighting filter based on this frequency transferresponse. The output of this filter 901 couples to a power detecting orsignal squaring device 902, which provides an output to a low-passfilter 903. A low-pass filter 903 is a device that receives a signalhaving a range of frequency components, attenuates frequency componentsabove a frequency threshold, and transmits frequency components belowthe frequency threshold.

An analog-to-digital converter (“ADC”) receives the low-pass filter'soutput and generates a corresponding digital signal which feeds into thedigital controller 905. The digital controller 905 in turn generatesdigital control signals for each of the adjustable delay 701 and thetapped delay line filter 600. Respective digital-to-analog converters(“DACs”) 906 a, 906 b convert these signals into the analog domain forrespective transmission over a delay control line 830 and a filtercontrol line 820. The analog delay control signal adjusts the adjustabledelay 701, while the analog filter control signal adjusts the tappeddelay line filter 600.

It will be useful to discuss a simple operational example in whichcrosstalk is imposed on a channel that is in a temporary condition ofnot carrying data. More specifically, consider a case in which thevictim transmitter 104 a does not transmit any data while the aggressingtransmitter 104 b is sending data with a broad spectral content or rangeof signal frequencies, such as pseudo-random or coded pseudo-randomdata. That is, referring briefly back to FIG. 5, the signal x(t) 214 isessentially zero, while u(t) 215 is a digital data signal having broadanalog spectral content resulting from randomly varying digital datapatterns. In this case, the signal y(t) 260 is simply the incurredaggressor n(t) 230, and the signal w(t) 520 is the modeled aggressor.Thus, the signal z(t) 420 is actually the modeling error of thecancellation device. In the theoretical and ideal situation of perfectcrosstalk cancellation, z(t) 420 is zero.

In other words, transmitting an essentially uniform voltage on thevictim channel 130 while transmitting signals having a broad range offrequencies on the aggressor channel 120 provides essentially purecrosstalk on the victim channel 130 and n(t) 230 equals y(t) 260. If thecrosstalk canceller 401 outputs a cancellation signal w(t) 520 alsoequaling the pure crosstalk signal n(t) 230, z(t) 420 will haveessentially no signal energy. Thus, in this state, signal energy in z(t)420 is indicative of modeling or delay inaccuracies in the crosstalkmodeling filter 501.

The control module 900 can implement this state of transmitting adefined signal on the aggressor channel 120 and transmitting a constantvoltage or essentially no data signal on the victim channel 130. Thecontrol module 900 can then adjust the adjustable parameters of thecrosstalk modeling filter 501′ to minimize the signal z(t) 420 receivedby the victim receiver 105 b, thereby providing a crosstalk cancellationsignal w(t) 520 that matches the actual crosstalk signal n(t) 230 andfurther providing a modeled crosstalk response G(f) 501 that effectivelymatches the actual crosstalk response H(f) 210. More generally, thecontrol module 900 cause transmission of defined or known signalpatterns on the aggressor channel 120, the victim channel 130, or boththe aggressor channel 120 and the victim channel 130 to characterize thecrosstalk effect 151 and to control, optimize, or adjust crosstalkcancellation or another form of crosstalk compensation. Further, thecontrol module 900 can have a learning or adaptive mode in the form of asetup mode or a self-configuration procedure and can implement automaticor self calibration.

Referring to FIG. 9 and generalizing beyond the example of imposingcrosstalk on a channel void of data, this error signal z(t) 420 can bespectrally weighted with an optional filter 901, whose response isdenoted as P(f), to emphasize any higher importance of certainfrequencies over others. For example, it may be desired to high-passfilter the error signal z(t) 420 to emulate the effect of equalizationin the victim receiver 105 b. The (potentially spectral weighted) errorsignal z(t) 420 is then squared or power-detected, i.e. the output ofthe squaring device 902 is the signal power. The power signal is thenpassed through a low-pass filter 903 (or integrator) with a relativelylow cutoff-frequency to obtain the integrated power, i.e. energy, of theerror signal z(t) 420. Thus, the signal at this point corresponds to ananalog estimate of the statistical variance (i.e. the square of thestandard deviation) of the error signal z(t) 420.

As familiar to those skilled in the art, the error variance is a usefulmetric for gauging fidelity. Because the cutoff frequency of thelow-pass filter 903 is at a very low frequency (typical orders ofmagnitude below the symbol transmission rate), the variance signal isnearly a constant after the transient effects of any modeling filterchanges decay away. Thus, the analog variance signal can be sampled witha simple low-speed high-resolution analog-to-digital converter 904. Thedigitized signal output by the analog-to-digital converter 904 providesthe error variance information to a simple microprocessor, statemachine, finite state machine, digital controller, or similar device(referred to herein as a “digital controller”) 905. After recording theerror-variance for the current set of response modeling parameters, thedigital controller 905 may then specify a new filter configuration bydigitally outputting the new parameters to a set of DACs 906 whichprovide the corresponding analog signal to the aggressor emulationmodule 501.

As the digital controller 905 is able to both (i) set the parameters ofthe crosstalk modeling filter 501 and (ii) directly observe the effectof the current parameters on the modeling error variance, the digitalcontroller 905 can find a parameter set that maximizes the fit of theaggressor response model 501 to the actual response 210. Becausetrial-and-error processing is not overly complicated, all combinationsof model parameters can be tested in many instances. However, otherempirical search/optimization methodologies known to those skilled inthe art may be alternatively employed. In one exemplary embodiment ofthe present invention, a coordinate-descent approach, as described inU.S. patent application Ser. No. 10/620,477, discussed above, providessearch and optimization to identify acceptable model parameters.

As discussed above, the control module 900 can comprise a combination ofanalog and digital circuitry to provide a practical controlimplementation. Filter 901 and power detecting device 902 collectivelyinput and output a high-speed analog signal. The low-pass filter 903takes as input a high-speed analog signal and outputs a low-speed analogsignal. Filter 901, power detecting device 902, and low-pass filter 903collectively take a projection of a high-speed signal onto a low-speedsignal by extracting the relevant statistical information from thehigh-speed signal and presenting it in a more concise form. The ADC 904takes this low-speed analog signal as an input and outputs acorresponding digitized approximation. Consequently, the controller 905receives and processes this low-speed digital signal. Because thedigital signal is low-speed, the associated processing circuitry is lesscomplex than would be required if the signal was high-speed. The digitalcontroller 905 outputs low-speed digital control signals to thedigital-to-analog converters 906 a, 906 b which in turn output low-speedanalog signals. As a result of tandem simple high-speed analogpreprocessing and low-speed digital processing, the control module 900provides signal analysis based on a powerful statisticalcharacterization and implements a robust control methodology withrelatively little circuit complexity, which are factors that canfacilitate practical crosstalk cancellation in high-speed communicationssystems.

While the illustration in FIG. 9 employs a power-detecting (or signalsquaring) device 902 to generate the error-variance, afull-wave-rectifier (which takes the absolute value of the signal) maybe used as an alternative. For an implementation based on afull-wave-rectifier, the output of the low-pass filter 903 will now nolonger correspond to error variance, but will nevertheless represent avalid fidelity criterion. In particular, it is the 1-norm of the errorsignal 420, and thus the fidelity metric still has suitable mathematicalproperties. Those skilled in the art appreciate that determining the“1-norm” of a signal typically comprises integrating the absolute valueof the control signal. This substitution may be advantageous for certainapplications because: (i) the 1-norm signal may have a reduced dynamicrange (thus relaxing resolution constraints on the analog-to-digitalconverter 904); and (ii) full-wave-rectifiers may be easier to implementthan power-detectors. Such modifications are considered within the scopeof the present invention.

Similarly, the power detector 902 may also be replaced with ahalf-wave-rectifier or any like device that is used to assess signalmagnitude. It will also be appreciated by those skilled in the art thatthe division of the crosstalk canceller 401 into functional blocks,modules, and respective sub-modules as illustrated in FIGS. 5 through 9are conceptual and do not necessarily indicate hard boundaries offunctionality or physical groupings of components. Rather,representation of the exemplary embodiments as illustrations based onfunctional block diagrams facilitates describing an exemplary embodimentof the present invention. In practice, these modules may be combined,divided, and otherwise repartitioned into other modules withoutdeviating from the scope of the present invention.

In one exemplary embodiment of the present invention, a crosstalkcancellation system is a single integrated circuit (“IC”), such as amonolithic IC. Each of a crosstalk cancellation device, a controlmodule, and a crosstalk modeling filter can also be single ICs. Such ICscan be complementary metal oxide semiconductor (“CMOS”) ICs and can befabricated in a 0.18 micron process, for example.

A process for canceling crosstalk and a process for calibrating acrosstalk canceller will now be described with respective reference toFIG. 10 and FIG. 11. Certain steps in the processes described hereinmust naturally precede others for the present invention to function asdescribed. However, the present invention is not limited to the order ofthe steps described if such order or sequence does not alter thefunctionality of the present invention. That is, it is recognized thatsome steps may be performed before or after other steps or in parallelwith other steps without departing from the scope and spirit of thepresent invention.

Turning now to FIG. 10, this figure is a flow chart illustrating Process1000, entitled Cancel Crosstalk, for canceling crosstalk 151 accordingto an exemplary embodiment of the present invention. At Step 1010, thefirst step in Process 1000, the aggressor transmitter 104 b transmits anaggressor communication signal u(t) 215 on the aggressor channel 120.This communication signal 215 can be an analog or a digital signalcarrying data.

At Step 1015, the crosstalk effect 151 couples energy from the aggressorcommunication signal u(t) 215 into the victim channel 130 as crosstalkn(t) 230. The coupling mechanism can be electromagnetic coupling, as inthe exemplary case of electrical data signals propagating on a backplane103, or another optical or electrical crosstalk mechanism. The energytransfer of the crosstalk effect 151 generates the crosstalk signal n(t)215 in the victim channel 130 in a manner that results in signalpropagation towards the victim receiver 105 b.

At Step 1020, the victim transmitter 104 a transmits the victimcommunication signal x(t) 214 on the victim channel 130. The victimcommunication signal 214 can be either an analog or a digital signal. AtStep 1025, the crosstalk signal n(t) 230 coexists or mixes with thevictim communication signal x(t) 214 in the victim channel 130. Thecomposite signal y(t) 260 results from the combination of these signals214, 230.

At Step 1030, the crosstalk model 501 acquires a sample of the aggressorcommunication signal u(t) 215. In other words, a tap or other nodedirects a representative portion of the aggressor communication signal215 to the crosstalk canceller 401 for reception and processing by thecrosstalk model 501.

At Step 1035, the crosstalk model 501 processes the sampled portion ofthe aggressor communication signal u(t) 215 via the tapped delay linefilter 600. Modeling parameters, such as the gain or scaling constantsof the tapped delay line filter 600, provide the basis for generating awaveform estimate v(t) 620 of the crosstalk signal n(t) 215. Morespecifically, the coefficients α₀, a₁, a₂ . . . α_(n) (alpha₀, alpha₁,alpha₂ . . . alpha_(n)) of the variable coefficient amplifiers 602 a,602 b, 602 c, 602 d in the tapped delay line filter define a waveformv(t) 620 approximating the crosstalk signal 215.

At Step 1040, the adjustable delay 701 in the crosstalk model 501applies a time delay to the waveform estimate v(t) 620 to synchronizethis waveform 620 with the interfering crosstalk signal n(t) 230propagating in the victim channel 130. At Step 1045, the summation node502 of the crosstalk canceller 401 applies the resulting crosstalkcancellation signal w(t) 520 to the victim channel 130 and the combinedcrosstalk and communication signal y(t) 260 propagating therein. Thecrosstalk cancellation signal w(t) 520 cancels at least a portion of thecrosstalk signal component w(t) 520 propagating in the victim channel130. Reducing this crosstalk interference 520 improves signal fidelityin the communication signal z(t) 420 that is output by the crosstalkcanceller 410 for delivery to the victim receiver 105 b.

At Step 1050, the controller 503 processes or analyzes the crosstalkcompensated signal z(t) 420 to determine effectiveness of crosstalkcancellation. In other words, the controller 503 assesses signalfidelity to determine if the crosstalk canceller is applying a crosstalkcancellation signal w(t) 520 that accurately matches the actualcrosstalk n(t) 230, both in waveform and in timing.

At Step 1055, the controller 503 adjusts the modeling parameters,specifically the coefficients of the variable coefficient amplifiers 602a, 602 b, 602 c, 602 d in the tapped delay line filter 600, to optimizethe waveform match between the crosstalk cancellation signal w(t) 520and the actual crosstalk signal n(t) 230. The controller 503 furtheradjust the variable or adjustable time delay of the adjustable delay 701to synchronize the crosstalk cancellation signal w(t) 520 with theactual crosstalk signal n(t) 230. That is, the controller 503 adjuststhe operation of the crosstalk canceller 401 by implementing parameteradjustments to the crosstalk modeling filter 501 to enhance the fidelityof the net communication signal z(t) 420 delivered to the victimreceiver 105 b.

Following Step 1055, Process 1000 iterates Steps 1010–1055. Thecrosstalk canceller 401 continues canceling crosstalk 230 andimplementing adaptive responses to dynamic conditions, thereby providingan ongoing high level of communication signal fidelity.

Turning now to FIG. 11, this figure is a flow chart illustrating Process1100, entitled Calibrate Crosstalk Canceller, for calibrating acrosstalk cancellation device 401 according to an exemplary embodimentof the present invention. At Step 1110, the first step in Process 1100,the controller 503 initiates a calibration sequence. The controller 900instructs the aggressor transmitter 104 b to output a signal having aknown or defined test pattern, for example a random or pseudo random bitpattern of data, onto the aggressor channel 120. This test orcalibration signal can have the format of an aggressor communicationsignal u(t) 215 or can be uniquely formatted for characterizing thecrosstalk response H(f) 210. That is, the controller 900 can controltransmission of signals having predetermined voltage patterns on theaggressor channel 120.

At Step 1115, the controller 900 instructs the victim transmitter 104 bto output a known victim test or reference signal onto the victimchannel 130. The test signal can be a predetermined communication signalor simply a constant voltage, null of data. Sending a known test signalon the victim channel 130 facilitates isolating the crosstalk responseH(f) 210 from other effects that may generate signal distortion on thevictim channel 130. That is, the controller 900 can control transmissionof signals having predetermined voltage patterns on the victim channel130.

At Step 1120, crosstalk n(t) 230 from the known aggressor signal u(t)215 couples into the victim channel 130. With the victim channel 130carrying a constant voltage as the victim signal x(t) 214, the compositecommunication and crosstalk signal y(t) 260 on the victim channel 130 isessentially the crosstalk signal n(t) 230.

At Step 1125, the crosstalk canceller 401 generates an estimate w(t) 520of the crosstalk signal n(t) 230 for crosstalk cancellation. Thecrosstalk canceller 401 generates this estimate 520 using modeling anddelay parameters that result in a waveform and timing match between thecrosstalk signal n(t) 230 and the crosstalk cancellation signal w(t)520. The crosstalk compensator 401 applies the crosstalk estimate 520 tothe victim channel 130 and cancels at least a portion of the crosstalk230 propagating thereon. The resulting crosstalk-cancelled signal z(t)420 propagates to the victim receiver 105 b.

At Step 1130, the controller 503 processes and analyzes thecrosstalk-cancelled signal z(t) 420 output by the crosstalk canceller401. Based on the analysis, the controller 503 adjusts the modeling anddelay parameters to minimize the energy in the crosstalk cancelledsignal z(t) 420. That is, the controller 503 varies the operationalparameters of the crosstalk canceller 401 towards reducing the residualcrosstalk. This control action matches the crosstalk compensation signalw(t) 520 with the actual crosstalk n(t) 230 imposed on the victimchannel 130.

At Step 1140, the controller 503 completes the calibration cycle andprovides notification to the aggressor and victim transmitters 104 a,104 b that the crosstalk canceller 401 is set to process live data. Inresponse to this notification, at Step 1145 the victim transmitter 104 aand the aggressor transmitter 104 b each transmit live data on theirrespective channels 130, 120.

At Step 1150, crosstalk 230 from live data 215 transmitting on theaggressor channel 120 couples into the victim channel 130. At Step 1155,the crosstalk canceller 401 processes a sample of the live data 215transmitting in the aggressor channel 120 and generates an emulation orestimate 520 of the crosstalk 230 using the modeling and delayparameters defined or updated during calibration.

At Step 1160, the crosstalk canceller 401 applies the crosstalk estimate520 to the victim channel 130 for crosstalk cancellation and presentsthe victim receiver 105 with a high-fidelity signal. Process 1100 endsfollowing Step 1160. The controller 503 can repeat the calibrationprocedure at defined or regular time intervals or when the controller'smonitoring capability determines that signal fidelity is impaired or asfallen below a threshold.

Turning now to FIGS. 12A and 12B, these figures respectively illustratetesting data of a communication system before and after implementingcrosstalk cancellation according to an exemplary embodiment of thepresent invention. These figures present eye diagrams 1200, 1250 ofmeasured data captured under laboratory conditions. As is well known tothose skilled in the art, eye diagrams 1200, 1250 provide a visualindication of signal quality. The level of openness of an “eye” 1225,1275 in an eye diagram 1200, 1250 correlates with the level of signalquality. That is, a noisy, distorted, or closed eye in an eye diagramtypically indicates signal impairment.

FIG. 12A is an eye diagram 1200 from a 5 Gigabits per second binarycommunication system operating under laboratory conditions believed tobe representative of field conditions. The victim signal 130 has anamplitude of 800 millivolts, while the aggressor signal 120 has anamplitude of 1,200 millivolts. FIG. 12A illustrates the eye diagram 1200of the received signal 260 after equalization and limiting amplificationbut without crosstalk compensation. FIG. 12B illustrates the eye diagram1250 of the received signal 420 after application of crosstalkcancellation in accordance with an exemplary embodiment of the presentinvention followed by equalization and limiting amplification. As withthe eye diagram of FIG. 12A, the victim signal 130 has an amplitude of800 millivolts, while the aggressor signal 120 has an amplitude of 1,200millivolts.

Because the signal path includes a limiting amplifier in both thecrosstalk-corrected eye diagram 1250 and the eye diagram 1200 withoutcrosstalk compensation, the thicknesses of the horizontal “eye-lids” atthe top and bottom of each eye diagram 1200, 1250 do not provide auseful gauge of signal quality. Rather the signal performanceenhancement provided by crosstalk cancellation is evident from the wideopen eye 1275 in the crosstalk-corrected eye diagram 1250 relative tothe narrow, noisy eye 1225 of the eye diagram 1225 without crosstalkcorrection.

To further characterize communication performance improvement achievedby crosstalk cancellation in accordance with an exemplary embodiment ofthe present invention, bit error rate measurements were acquired fromthis test system under the same test conditions, before and aftercrosstalk cancellation. Without crosstalk cancellation, thecommunication system exhibited an average of one bit error for every100,000 bits transmitted. With crosstalk cancellation, the communicationsystem exhibited an average of one bit error for every100,000,000,000,000 bits transmitted.

Although a system in accordance with the present invention can comprisea circuit that cancels, corrects, or compensates for crosstalk imposedon one communication signal by another signal, those skilled in the artwill appreciate that the present invention is not limited to thisapplication and that the embodiments described herein are illustrativeand not restrictive. Furthermore, it should be understood that variousother alternatives to the embodiments of the invention described heremay be employed in practicing the invention. The scope of the inventionis intended to be limited only by the claims below.

1. A signal processing system, for applying a crosstalk estimate to afirst communication channel to compensate for crosstalk coupled into thefirst communication channel from a second communication channel,comprising: a model, coupled to the second communication channel,operative to generate the crosstalk estimate, comprising: a waveformshaper comprising a plurality of delay stages coupled to a plurality ofrespective amplifiers; and an adjustable delay coupled to the waveformshaper; and a controller, operative to process crosstalk compensatedcommunication signals and adjust the waveform shaper and the adjustabledelay.
 2. The signal processing system of claim 1, wherein the waveformshaper comprises a tapped delay line filter.
 3. The signal processingsystem of claim 1, wherein the adjustable delay receives input from thewaveform shaper.
 4. The signal processing system of claim 1, wherein theadjustable delay provides input to the waveform shaper.
 5. The signalprocessing system of claim 1, wherein the model further comprises afixed filter, coupled to at least one of the waveform shaper and theadjustable delay, operative to transmit frequencies above a thresholdfrequency and attenuate frequencies below the threshold frequency. 6.The signal processing system of claim 1, wherein the controllercomprises a digital controller.
 7. The signal processing system of claim1, wherein the controller is further operative to control transmissionof signals having predetermined voltage patterns on the first and secondcommunication channels.
 8. The signal processing system of claim 1,wherein the controller is operative to calibrate the model.
 9. A methodfor processing a first communication signal having crosstalk from asecond communication signal comprising the steps of: sampling the secondcommunication signal; processing the sample of the second communicationsignal to generate a crosstalk emulation signal; applying the crosstalkemulation signal to the first communication signal to cancel at leastsome of the crosstalk; generating a third communication signal based onthe application of the crosstalk emulation signal; analyzing the thirdcommunication signal; and adjusting the crosstalk emulation signal basedon the analysis, wherein the adjusting step comprises timing thecrosstalk emulation signal to match the crosstalk.
 10. The method ofclaim 9, wherein: the processing step comprises processing the samplewith a model comprising at least one modeling parameter; and theadjusting step further comprises adjusting the at least one modelingparameter.
 11. The method of claim 9, wherein: generating the crosstalkemulation signal comprises generating a waveform differing from thecrosstalk by a difference; and the adjusting step further comprisesreducing the difference.
 12. The method of claim 9, wherein: analyzingthe third communication signal comprises monitoring for residualcrosstalk in the third communication signal; and adjusting the crosstalkemulation signal further comprises reducing the residual crosstalk. 13.The method of claim 9, wherein applying the crosstalk emulation signalto the first communication signal comprises subtracting the crosstalkemulation signal from the first communication signal.
 14. A method forprocessing a first communication signal having crosstalk from a secondcommunication signal comprising the steps of: sampling the secondcommunication signal; processing the sample of the second communicationsignal to generate a crosstalk emulation signal; applying the crosstalkemulation signal to the first communication signal to cancel at leastsome of the crosstalk; generating a third communication signal based onthe application of the crosstalk emulation signal; analyzing the thirdcommunication signal; and adjusting the crosstalk emulation signal basedon the analysis, wherein: the processing step comprises synchronizingthe crosstalk emulation signal with the crosstalk to a level ofsynchronization; and the adjusting step comprises improving the level ofsynchronization.
 15. A signal processing method, comprising the stepsof: transmitting a test signal over a first communication channel;coupling a portion of the test signal from the first communicationchannel into a second communication channel via a crosstalk effect;defining a model of the crosstalk effect based on processing the portionof the test signal coupled into the second communication channel via thecrosstalk effect; transmitting a first communication signal over thefirst communication channel; coupling a portion of the firstcommunication signal into the second communication channel via thecrosstalk effect; transmitting a second communication signal over thesecond communication channel; processing a portion of the firstcommunication signal with the model and outputting an estimate of theportion of the first communication signal coupled into the secondcommunication signal via the crosstalk effect; and compensating for thecrosstalk effect by applying the estimate to the second communicationchannel.
 16. The method of claim 15, wherein the step of transmittingthe test signal over the first communication channel further comprisestransmitting data on the first communication channel and providing anessentially uniform voltage on the second communication channel.
 17. Themethod of claim 15, wherein defining the model of the crosstalk effectcomprises adjusting a modeling parameter of the model.
 18. The method ofclaim 15, wherein defining the model of the crosstalk effect comprisesadjusting a signal delay of the model.
 19. The method of claim 15,wherein defining the model of the crosstalk effect comprises setting again of the model.
 20. The method of claim 15, wherein applying theestimate to the second communication channel comprises subtracting theestimate from the second communication signal and the portion of thefirst communication signal coupled into the second communication channelvia the crosstalk effect.
 21. A signal processing system, for correctingcrosstalk coupled from a first communication channel into a secondcommunication channel, comprising: a modeling filter, coupled to thefirst communication channel, operative to generate an estimate of thecrosstalk, comprising: an adjustable analog filter, controllable by amodeling input; and an adjustable delay coupled to the adjustable analogfilter; and an analysis circuit, operative to analyze crosstalkcorrected signals and adjust the adjustable analog filter and theadjustable delay.
 22. The signal processing system of claim 21, whereinthe analog filter comprises a tapped delay line filter.
 23. The signalprocessing system of claim 21, wherein the adjustable delay receivesinput from the analog filter.
 24. The signal processing system of claim21, wherein the adjustable delay provides input to the analog filter.25. The signal processing system of claim 21, wherein the modelingfilter further comprises a fixed filter, coupled to at least one of theanalog filter and the adjustable delay, operative to transmitfrequencies above a threshold frequency and attenuate frequencies belowthe threshold frequency.
 26. The signal processing system of claim 21,wherein the analysis circuit comprises a digital controller.
 27. Thesignal processing system of claim 21, wherein the analysis circuit isfurther operative to control transmission of signals havingpredetermined voltage patterns on the first and second communicationchannels.
 28. The signal processing system of claim 21, wherein theanalysis circuit is further operative to calibrate the modeling filter.29. An apparatus, for correcting crosstalk in a data communicationssystem, comprising: a crosstalk modeling filter that models a crosstalkresponse; a difference node that subtracts the modeled crosstalkresponse from a communication signal, comprising the crosstalk, to yielda crosstalk-compensated signal, and a control circuit that processes thecrosstalk-compensated signal and adjusts the crosstalk modeling filterto reduce residual crosstalk in the crosstalk-compensated signal. 30.The apparatus of claim 29, wherein the crosstalk modeling filtercomprises an adjustable delay filter and an analog tapped delay linefilter.
 31. The apparatus of claim 30, wherein the adjustable delayfilter circuit comprises a digital circuit that receives a data signalcausing the crosstalk and amplitude limits the data signal.
 32. Theapparatus of claim 30, wherein the crosstalk modeling filter furthercomprises a first-order high-pass filter.
 33. The apparatus of claim 29,wherein the control circuit comprises an analog low-pass filter, ananalog-to-digital converter, and a digital controller.
 34. The apparatusof claim 33, wherein the data communications system communicates data ata baud rate and the analog-to-digital converter operates at a speedlower than the baud rate.
 35. The apparatus of claim 33, wherein thedata communication system transmits data at a baud rate and the digitalcontroller operates at a speed less than baud rate.
 36. The apparatus ofclaim 33, wherein a device that measures signal magnitude based onsignal amplitude provides input to the analog low-pass filter.
 37. Theapparatus of claim 36, wherein the device comprises a power-detector.38. The apparatus of claim 36, wherein the device comprise a full-waverectifier.
 39. The apparatus of claim 36, wherein the device comprises ahalf-wave rectifier.
 40. The apparatus of claim 36, wherein a spectralweighting filter provides input to the device.
 41. The apparatus ofclaim 33, wherein the control circuit comprises a finite state machine.42. The apparatus of claim 33, wherein the control circuit comprises amicroprocessor.